The production of three-dimensional articles of complex shape by means of stereolithography has been known for a relatively long time. In this technique the desired shaped article is built up from a liquid, radiation-curable composition with the aid of a recurring, alternating sequence of two steps (a) and (b); in step (a), a layer of the liquid, radiation-curable composition, one boundary of which is the surface of the composition is cured with the aid of appropriate radiation, generally radiation produced by a preferably computer-controlled laser source, within a surface region which corresponds to the desired cross-sectional area of the shaped article to be formed, at the height of this layer, and in step (b) the cured layer is covered with a new layer of the liquid, radiation-curable composition, and the sequence of steps (a) and (b) is repeated until a so-called green model of the desired shape is finished. This green model is, in general, not yet fully cured and must therefore, normally, be subjected to post-curing.
The mechanical strength of the green model (modulus of elasticity, fracture strength), also referred to as green strength, constitutes an important property of the green model and is determined essentially by the nature of the stereolithographic-resin composition employed. Other important properties of a stereolithographic-resin composition include a high sensitivity for the radiation employed in the course of curing and a minimum curl factor, permitting high shape definition of the green model. In addition, for example, the precured material layers should be readily wettable by the liquid stereolithographic-resin composition, and of course not only the green model but also the ultimately cured shaped article should have optimum mechanical properties. A still further important property of the cured article is increased flexibility and toughness, as measured by elongation at break and Izod impact resistance.
Liquid, radiation-curable compositions for stereolithography which meet at least some of the abovementioned requirements are described, for example, in U.S. Pat. No. 5,476,748, which is incorporated herein by reference. The so-called hybrid systems compositions shown therein comprise free-radically and cationically photopolymerizable components. Such hybrid compositions comprise at least:
(A) a liquid difunctional or more highly functional epoxy resin or a liquid mixture consisting of difunctional or more highly functional epoxy resins; PA0 (B) a cationic photoinitiator or a mixture of cationic photoinitiators; PA0 (C) a free-radical photoinitiator or a mixture of free-radical photoinitiators; and PA0 (D) at least one liquid poly(meth)acrylate having a (meth)acrylate functionality of more than 2, PA0 (E) at least one liquid diacrylate, and PA0 (F) a polyol component selected from the group consisting of OH-terminated polyethers, polyesters and polyurethanes. PA0 A.sup.- is CF.sub.3 SO.sub.3 or an anion of the formula [LQ.sub.mB ], where PA0 L is boron, phosphorus, arsenic or antimony, PA0 Q is a halogen atom, or some of the radicals Q in an anion LQ.sub.m.sup.- may also be hydroxyl groups, and PA0 mB is an integer corresponding to the valency of L enlarged by 1. PA0 dB is 1, 2, 3, 4 or 5, PA0 XB is a non-nucleophilic anion, especially PF.sub.6.sup.-, AsF.sub.6.sup.-, SbF.sub.6.sup.-, CF.sub.3 SO.sub.3.sup.-, C.sub.2 F.sub.5 SO.sub.3.sup.-, n-C.sub.3 F.sub.7 SO.sub.3.sup.-, n-C.sub.4 F.sub.9 SO.sub.3.sup.-, n-C.sub.6 F.sub.13 SO.sub.3.sup.- and n-C.sub.8 F.sub.17 SO.sub.3.sup.-, PA0 R.sub.8B is a .pi.-arene and PA0 R.sub.9B is an anion of a .pi.-arene, especially a cyclopenadienyl anion.
Such hybrid systems can optionally further contain vinyl ether-based resins or other cationically cured components such as oxetanes, spiro-ortho esters.
It is known to a person skilled in the art that the majority of commercial hybrid stereolithography compositions suffer from very low elongation at break and Izod impact resistance. Their average values are around 4% and 0.45 ft.lb/in, respectively. Cured objects resulting from these compositions are very brittle, and not very functional for rapid prototyping and verification type applications.
Attempts have been made to solve the problem of brittleness of cured articles produced from stereolithography compositions, in general. To date, the efforts have concentrated on acrylate-based compositions, which use acrylate urethane oligomers or their relatively low molecular weight polymers as flexibilizers. It is well known that urethane acrylate oligomers or polymers are highly flexible and tough (high Izod impact resistant) materials. Incorporation of these molecules into acrylate compositions, accordingly, makes the cured objects flexible and tough (durable). These efforts employ specific diluents, such as urethane acrylates, as shown in European patent application 562,826 to Loctite Corp., monomeric or oligomeric aliphatic urethanes, as shown in German patent application DE 4,138,309, to EOS GmbH (Electro Optical Systems), monofunctional diluting monomers, as shown in Japanese patent application 97-431498, to Mitsubishi Rayon Co., Ltd., and an unsaturated urethane, as shown in Japanese patent application 97-466285, to Takemoto Oil & Fat Co., Ltd. Cured objects resulting from such compositions are flexible and show relatively high impact resistance, sometimes close to 1-1.3 ft.lb/in.
A major disadvantage of flexibilized acrylate urethane compositions is the fact that: 1) polymerization is hindered by atmospheric oxygen because the polymerization thereof is of radical nature; 2) the cure shrinkage is unacceptably large; and 3) the acrylate urethane compounds are irritant to the skin, particularly when the viscosity is low (low viscosity is highly preferred for stereolithography applications). Attempts to incorporate acrylate urethane flexibilizers into hybrid stereolithographic compositions have been unsuccessful as a means for improving flexibility and toughness. This approach tends to reduce photospeed to unacceptable levels. This reduction in photospeed is due to the fact that the acid generated from the disassociation of the cationic photoinitiator, which is responsible for the polymerization of the epoxy ring or any other cationically cured compound, reacts with the nitrogen of the urethane group, thus prohibiting the cationic photopolymerization reaction from taking place.
On the other hand, while cationic or hybrid cationic-radical stereolithographic compositions can moderate the effects of the aforementioned problems of the acrylate chemistry, the cured objects are very brittle and show very low toughness.
An alternative technique that has been used for improving the flexibility of hybrid stereolithography compositions entails the use of low to medium range molecular weight diols triols or polyols, especially polyether polyols. This approach has been employed for years, and is still being used. It relies on the reduction of the crosslink density of the three dimensional network as a means of reducing the brittleness of a cured object. As an example, a recent worldwide patent application on photo-curable stereolithographic resin compositions to DSM Corp, Japan Synthetic Rubber Co., and Japan Fine Coatings, Co., Ltd., WO 97/38354 (Oct. 16, 1997) teaches the use of polyether polyols to increase the extensibility and toughness of the three-dimensional object. These polyether polyols have on average about 3 or more hydroxyl groups in one molecule. Examples of the suggested polyether polyols include original or ethyleneoxide chain-extended glycerol, pentaerythiritol, trimethylolpropane, sorbitol, sucrose.
The preferable molecular weight of the polyether polyol is about 100 to 2000, and more preferably about 160-1000. This patent also teaches that "if polyether polyols having too large molecular weight are used, it results in lowering the mechanical strength of the three-dimensional object obtained by the photo-fabrication process".
However, major drawbacks of the polyether polyol flexibilizing method are 1) drastic reduction of thermal properties such as heat deflection temperature, glass transition temperature, 2) reduction of the rigidity of the cured article and 3) reduction of water and humidity resistance of the cured object. Despite all previous attempts, there exists a need for a stereolithography composition capable of producing a flexible and durable cured article for which the photospeed, and cure depths are commercially acceptable.